Typically, autonomous vehicles perform localisation processes to navigate.
Many conventional autonomous vehicles use an Inertial Navigation System (INS) for localisation.
Typically, a combination of high-error, low-drift sensors and low-error high-drift sensors is used. An example of such a high-error, low-drift sensor is the Naystar Global Positioning System (GPS) which may be used to measure (global) positions for the vehicle. An example of a low-error, high-drift sensor is an Internal Measurement Unit (IMU) which may be used to measure turn rates, velocities and accelerations etc. for the vehicle.
However, an INS tends not to be robust to failure/error of the GPS. This tends to be because without the global position of the vehicle (determined by the GPS), linear speeds of the vehicle tend to be unobservable and, over time, an error associated with this value can become very large.
Thus, the performance of an autonomous vehicle tends to be adversely effected in environments in which a GPS signal is intermittent, or not present. Moreover, to keep the vehicle moving safely in such environments it tends to be necessary for the vehicle's localisation system to keep within the performance specification required by the sensor fusion and control systems. For example, if error(s) in speeds became greater than those permitted the perception and control systems of the vehicle, the vehicle will tend no longer to be able to see or steer through the world reliably. Therefore, the motion of the vehicle tends to be unsafe.